Threaded connections



NOV. 28, 1967 J KLOESEL, JR ET AL 3,355,192

THREADED CONNECTIONS 2 Shets-Sheet l r w J j q 1 MW em JO Tl, M MM a 0A P Q mm J 6 I L 5 F'ileq Aug.

y/ia/ ATTOR/Vf r Nov. 28, 1967 J, A. KLOESEL, .JR.. ET AL 3,355,192

THREADED CONNECTIONS I Filed Aug. 9, 1965. 2 Sheets-Sheet 2 Jam 7. CrewJ (fare 00 A. /(/0e$e/, Jr.

INVEN'JOR.

ATTO/WVf) United States Patent 3,355,192 THREADED CONNECTIONS Joseph A. Kloesel, Jr., and Sam T. Crews, Midland, Tex.,

assignors to Drilco Gil Tools, Inc, Midland, Tex., a corporation of Texas Filed Aug. 9, 1965, Ser. No. 494,996 3 Claims. (Cl. 285--94) This invention pertains to tapered threaded connections, more particularly rotary shouldered connections for drill pipe used in mining. Such connections are taper threaded so that they can be made up tight enough to prevent leakage, wobble, and unscrewing. Shoulders or other abutments are provided to limit make up due to creepage during use, thereby to prevent bursting of the box and collapse of the pin. After the shoulders engage, further makeup requires that the pin root be stretched and the end of the box be compressed. Due to the resultant pressure at the faces of the shoulders the shoulers also provide a primary seal and reduce the bending moment stresses, especially on the pin. 7

The connection embodying the invention is particularly adapted for connecting lengths of drill pipe used in boring mine raises.

(a) Mine raises Mine raises are large diameter inclined bores connecting tunnels at different levels and used to transfer ore and earth by gravity to the lowermost tunnel from the tunnels thereabove, all the ore and earth then being removed from the lowest tunnel by a suit-able conveyor.

(b) Bit loading tension Mine raise bores are typically four feet in diameter. In order to put enough axial load on the drill pipe to create enough pressure on a 48 inch bit to bore through rock or other hard earth formations it is necessary to bore with the drill pipe in tension; the requisite load placed on a drill pipe in compression would cause it to buckle unless the pipe were oversize and consequently excessively heavy. In order to bore a hole with the drill pipe in tension it is necessary first to bore in compression a rat hole, Le. a preliminary smaller diameter hole through which drill pipe large enough to drive the drill bit for the tension bored hole can pass. The rat hole is bored in compression and the final hole is bored in tension.

(c) Weight tension The drill, i.e. the machine for rotating the drill pipe and controlling its axial motion, can be at either the upper level or the lower level for boring the rat hole, and a like choice exists for boring the final hole. Absent any reason for moving the drill from one level to another, it is of course more economical to leave it in one position. It is preferred to bore withthe drill located at the upper level so that cuttings and water falling down the hole will not drop onto the drill and the men operating it. Therefore, the rat hole is bored in compression from the upper level down to the lower level, following which the final hole is bored in tension from the lower level up to the upper level. Since the final hole is bored upwardly, the weight of the drill pipe which may be of the order of 10,000 pounds (200 ft. at 50 lb. per foot) and the weight of the drill bit, which may be about a ton, thus place the drill pipe in tension even before any axial bit loading tension is applied by the drill.

(d) Connections initially merely hand tight The drill pipe used in mines is usually five feet long. Space restrictions underground prevent the use of longer pipes. For the same reason, space restrictions, elaborate equipment such as long tongs and power driven cable winches are not usually used in mine drilling. The pipe 3,355,192 Patented Nov. 28, 1967 connections are therefore made up only hand tight to begin with and the torque of drilling allowed to make up the connections more fully. In the case of the tension boring of the final hole the drill pipe string will be made up full length to start with and thereafter will be shortened as the bit progresses toward the drill; therefore once the connections have made up tight at the commencement of the boring operation, they will be fluid tight and there will be no problem with leakage due to the connections having initially been made up only hand tight. Furthermore, during the upward drilling of the final hole, the only fluid that the drill pipe need conduct is air used to clean the bit bearings; the cuttings fall by gravity to the bottom of the hole and are removed by a conveyor, so it is unnecessary to use water, mud, or other high pressure liquid in the drill pipe to remove the cuttings. On the down boring operation, it may be necessary to use a liquid rather than air or other gas to remove the cuttings but the pressure is not high since the length of a mine raise is usually only a few hundred feet or less. For these reasons also, initial make-up of the drill pipe merely hand tight is permissible.

(e) Drilling torque and make-up Due to the large size bit, e.g. 48 inches, and the heavy axial loading of the bit, which may approach the full capacity of the drill, e.g. 350,000 pounds, the torque required to rotate the drill pipe is very high, e.g. 80,000 lb. ft., resulting in extremely tight make-up. Another factor affecting make-up is the fact that the combined tension of weight of drill pipe and bit plus axial pull of the drill stretches the root of the pin of each threaded connection tending to cause separation of the face of the box and the shoulder on the pin, thus allowing more makeup.

Two problems occur if make-up is excessive. First of all, when tapered connections are made up tight the box is placed in tension and the pin in compression. Excessive make-up tends to burst the box, collapse the pin, and to strip and gall the threads.

(1) Breakout torque Secondly, breakout torque is a function of make-up torque andif the make-up torque is high the breakout torque will also be high. As noted previously, special equipment such as power winches and long tongs, which could be used to facilitate breakout, is usually unavailable at a mine drill. Furthermore, even if the drill were equipped with a power winch it would be unavailable at the bottom of the hole where the 48 inch bit must be connected to the drill pipe for the upward tension bore. Therefore, if the drill pipe connections are subjected to excessive make-up torque, difficulty may be experienced breaking the connections with the relatively short, e.g. four foot long, hand operated tongs or winches available in a name.

It may be noted here in passing that a high breakout torque is not a requirement for mine raise drill pipe, for due to the relatively short lengths of drill strings employed, e.g., a few hundred feet, the pipe does not wind up very much between the drill and bit; the pipe is therefore not subjected to any large reverse torque due to inertia when the pipe unwinds. This is a further reason why it is permissible to make-up the pipe initially merely hand tight.

(g) Dirt in connections Since mine raise drill pipe is assembled underground, mine raise drill pipe connections are particularly subject to the hazard of dirt being trapped between the threads and between the pin shoulder and box face or shoulder. Even if the threads and shoulders are initially clean when first made up, when the pipe is placed in the bore after being made up only hand tight, the tension due to the weight of the pipe and the drill bit may stretch the pin root enough to open up a crack between box face and pin shoulder into which dirt can enter.

(h) Bending moment Even drilling in tension there are unusual bending moments imposed on a drill pipe carrying a 48 inch bit. The relatively slim rat hole will limit lateral displacement of the drill pipe but repetitive bending stress will still be imposed on the threaded dril pipe connections tending to cause fatigue failure. Since the drill pipes are only five feet long, there will be forty threaded connections in a 200 foot drill string, all subjected to repetitive bending moments.

It will be apparent from the foregoing that there are unusual problems to be overcome in constructing a satisfactory threaded connection for drill pipe used in boring mine raises. The general object of the present invention is to provide a satisfactory connection for such usage.

More specifically the object of the invention is to provide a threaded connection which can be used successfully with the threads dirty and the connection subjected to extreme make-up torque without the pin collapsing and the box bursting and without the threads stripping or galling and which at the same time will have a quick make-up and breakout and a breakout torque within the limited capacity of the underground tools and equipment available at a mine raise drill, and which can withstand the combined tension loading, repetitive bending moments and underground mining corrosive conditions.

Other objects and advantages of the invention will appear from the following description of a preferred embodiment thereof.

The connection according to the invention comprises a novel combination of elements. Briefly the connection includes a tapered threaded pin having a shoulder at the large diameter part of the pin and a tapered threaded box gaged so that the face or shoulder at the end of the box engages the pin shoulder when made up only hand tight. The taper of the pin and box is steep, e.g., 1 /2 inches per foot. Coarse double threads are used, each having a large lead helix angle, e.g., with a lead of about 1" (two threads per inch double lead). The pin threads have a flank angle on the sides nearest the pin shoulder and a 45 flank angle on the other side. The box threads have a 45 flank angle on the sides nearest the pin shoulder and a 15 flank angle on the other side. None of the thread crests bottom in the grooves between the opposite threads and only the 15 degree flank angle thread flanks engage, leaving a large helical space between pin and box. The pin thread runs out into a tapered stress relief groove adjacent the pin shoulder; the box thread vanishes into the cylindrical inner periphery of the drill pipe; there are only about two full turns of complete box thread that are engaged with pin thread. The threads are Kem- Plated, i.e., treated with an acid producing an iron phosphate layer at the surface.

The resulting connection has a breakout torque that is only about 60% of the make-up (drilling) torque, e.g., the breakout torque may be about 50,000 lb. ft.

The ratio of breakout torque to make-up torque, is believed to be the result of the operation of a number of factors. The pin thread flank may be thought of as an inclined plane wrapped around the pin up which the pin thread flank slides as the connection is made up. The angle of inclination is the lead helix angle. During makeup, after the box end face engages the pin shoulder the mouth of the box is compressed axially and the root of the pin stretched axially as the box thread moves farther along the incline of the pin thread. The axial stresses thus produced create axial pressure between the thread flanks and between the pin shoulder and box end face. This results in frictional resistance to rotation of the box on the pin. During make-up there must be suflicient torque applied to compress the box and stretch the pin as well as overcome the friction between pin shoulder and box end face and between the thread flanks.

Absent any threads, the pin and box would appear as smooth comically tapered surfaces. The generatrix line of each conical surface is inclined relative to the cone axis so the cone surfaces may be thought of as inclines, the angle of inclination being the taper angle. If there is engagement of the pin or box thread crests with the interthreacl area of the box or pin, or if the engaged thread flanks are other than perpendicular to the axis of the connection, then as the connection is made up and the box moves up the incline of the pin taper, the box is placed in hoop tension and the pin is subjected to circumferential compression. This causes radial pressure between the thread flanks resulting in frictional resistance to rotation of the box on the pin. As the connection is made up, enough torque must be applied to expand the box and compress the pin and also to overcome the friction between any radially engaged surfaces of the threads. This is in addition to the aforementioned torque required to create the axial stress and overcome the friction resulting therefrom.

There are thus two inclines involved: one measured by the lead helix angle of the thread and the other measured by the taper of the pin and box; make-up of each involves both stress of the metal and a resultant frictional resistance, both of which must be overcome during makeup. When the connection is broken apart, the metal stresses acting against the inclines tend to unscrew the connection. This tends to make breakout torque less than make-up torque.

On the other hand the terminal position of make-up is the result of relative movement of pin and box so that the frictional resistance overcome is sliding friction. When the connection is to be broken the frictional resistance to be overcome is static friction, which usually is considerably greater than sliding friction. This tends to make breakout torque greater than make-up torque.

By using steep inclines, e.g. large taper pin and box and large lead helix angle threads, the hoop stresses on pin and box and the pin root tension and box mouth compression for any given make-up torque are reduced, so that the connection can better withstand high drilling torque. This also has the effect of increasing the ratio of torque needed to stress the metal (which tends to help when the connection is unscrewed) to the torque needed to overcome frictional resistance, so that the ratio of breakout to make-up torque is reduced.

A limitation is imposed, however, on the degree of taper of the pin and box. To achieve balance between the pin and box it is desired that the section modulus of the box just beyond the end of the pin be about two to three times the section modulus of the pin near its shoulder. For example, in the preferred embodiment to be described the box section modulus inch beyond the end of the pin is 2 /2 times the pin section modulus 4 inch from the pin shoulder. If the pin and box taper is too large, the pin bore must be unduly small in order to provide enough pin wall thickness to achieve the desired section modulus. Other factors, of course, limit the lead helix angle of the thread. There is therefore a limit on the degree of reduction of the ratio of breakout to make-up torque that can be achieved by increase of taper and increase of lead helix angle.

By eliminating contact between thread crests of both pin and box with the interthread space on the adjacent box and pin, and by using steep flank angles on the engaged thread flanks while leaving a large space between the unloaded thread flanks, there is provided a large open space into which dirt can move without damaging the threads. This also eliminates most of the friction area associated with hoop stress in the pin and box. The ratio of breakout to make-up torque is therefore reduced by this means also.

The Kent-Plating or phosphatizing of the threads in using a double thread the necessary strength in tension is achieved without the necessity of a large number of engaged thread turns on each thread, thereby further facilitating quick make-up. The short length of the section of the connection having fully engaged threads increases the flexibility of the threaded portion of the connection thereby reducing the bending moments at other portions of the connection. The pin thread runout into a groove and the vanishing box threads not only reduce stress concentrations but reduce the number of engaged thread turns as needed to increase flexibility.

It is thus apparent that the several features of the connection combine in multiple ways to produce the desired results.

For a more detailed description of the invention reference will now be made to the accompanying true scale drawings wherein FIGURE 1 is an elevation, partly in section, showing a threaded connection embodying the invention, and

FIGURE 2 is a sectional fragmentary view to a larger scale showing the details of the pin thread configuration. Referring now to FIGURE 1 there is shown a threaded connection between tubular steel members and 11. Members 10 and 11 may be the ends of two lengths of drill pipe or they could be short lengths of pipe Welded or screwed onto the ends of lengths of drill pipe, or they could be other tubular members desired to be connected by the threaded connection embodying the inven tion.

Member 10 has a pin 12 formed on an internal upset at the end thereof. On the outer periphery of pin 12 are formed two helical threads 13, 14. 'Pin 12 is externally tapered in the range of 1 A to 1% inches diameter per foot, preferably 1 /2" per foot as shown, and has a maximum outer diameter considerably less than the outer diameter of member 10 forming a radial shoulder perpendicular to the axis of the member. Between shoulder 15 and the adjacent ends of threads 13 and 14 there is a tapered stress relief undercut area 16. The area 16 is tapered the same as the threaded part of pin 12 and is undercut' inch indiameter below the threaded part of pin 12.

Member 11 has a" box 9 at one end thereof tapered the same as pin 12. The inner periphery of the box is unthreaded over an area 8 opposite the unthreaded undercut area 16"at' the root of the pin 12. This unthreaded portion of the box forms the mouth 21 of the box. The remainder of the box is provided with two helical threads 7 and '6 extending from the mouth of the box to the juncture of the tapered bore of the box with the cylindrical inner periphery of member 11. The crests of the first turns of the box threads adjacent the mouth of the box are tapered but thereafter they vanish' into the cylindrical surface defined by the inner periphery of member 11.

The dimensions of the pin threads are marked on FIG- URE 2, from which it is apparent that the width of the thread crests is less than the width of the space on the pin between adjacent thread roots. In other Words, the threads are narrower than the grooves. The box threads are of similar construction whereby there are large helical spaces 17, 18 between the unloaded thread flanks.

The length of the pin root 20 and the box mouth 21 are related to the thread crest cones so that when the pin shoulder 15 and end face 22 of the box are engaged, even after full make-up, there are still helical spaces 23, 24 between the crests of the pin threads and the adjacent box grooves and also helical spaces 25, 26 between the crests of the box threads and the adjacent pin grooves (interthread root areas). Specifically, to achieve this spacing, the box end face 22 is farther from the apex of the box thread crest cone than the pin shoulder 15 is from the apex of the pin thread root cone; also the box end face 22 is farther from the apex of the box thread root cone than the pin shoulder 15 is from the apex of the pin thread crest cone.

The spaces 25, 26 become larger nearer the end of the pin due to the threads on the box vanishing into the cylindrical inner periphery 28 of the pipe. The spaces 17, 23, 25 are in communication with each other forming one helical space. The spaces 18, 24, 26 are also in communication with each each other forming another helical space. It will be seen that it is only the engagement of the loaded 15 degree angle flanks 30, 31, 32, 33 of the threads that prevents the box from wobbling on the pin. The 15 degree angle, though small enough to keep hoop stress within reasonable limits is yet large enough to prevent wobble. The flank angle may be varied within the range of about 10 to 20 degrees.

The buttress form of the threads, that is, the 45 degree une-ng-aged flanks, provides adequate strength to the threads to take the axial loads imposed not only by the tension of the pipe but also due to bending moment tending to cause (but not actually causing) the box thread to slide laterally relative to the pin thread. Considerable latitude is permissible in the angle of the uneng aged thread flanks, eg 30 degrees to 60 degrees. Preferably the thread cross section is such that the thread height is of the same order of magnitude as the average width.

Shoulder 15 and end face 22 are bevelled off at 35, 36 in order to concentrate the axial load therebetween and provide a better seal. The bevels also reduce the likelihood of galls being formed at theouter periphery of the shoulder and end face by mechanical mistreatment, thereby further reducing the likelihood of leakage and poor engagement.

The thread lead of 1 inch per foot may be varied over a range, e.g. inch per foot to 1% inch per foot.

It is to be noted that the pin root 20 and box mouth 21 have a length of 1" from the shoulders 15, 22 to the commencement of the 30 box bevel and the 45 pin bevel leading up to the thread crest cones, the axial extent of the bevels being inch. The pin root and box mouth therefore have a length which is the same order of magnitude as the lead of the thread. This length is suflicient to allow enough pin root stretch and box mouth compression after initial engagement of shoulder and end face to keep the connection tight without undue thread deformation.

Computation shows that the clearance provided between the thread crests and interroot spaces, which is .O0 5.inch r-adius when the pin shoulder and box end face are made up hand tight, as shown in FIGURE 1, is sufficient with the taper of /1 inch radius per foot of length to allow .08 inch axial travel without the crests engaging with the interroot spaces. Actually, a little more travel can .occur without such engagement due to hoop stress in the pin and box. With engagement occurring only after a make-up of .08 inch, it may be said that substantial clearance remains after make-up of the order of 5% of the lead.

The foregoing computation is made on the basis that radial clearance axial travel to engagement; taper QQLZ? .08 12 having an outer diameter in the range of 4 inches to 12 inches. The nominal diameter of the pin, i.e. the diameter of the projection of the thread crest cone on the pin shoulder, may be varied according to the pipe diameter in order to achieve the desired balance of section modulus while maintaining a satisfactory pin borediameter. For example, a 4%" nominal pin diameter can be used with 6% OD. pipe; a 5%" nominal pin diameter can be used with 6%" or 7'' OD. pipe; a 6% nominal pin diameter can be used with 8" pipe. In all sizes of pipe, however, the same pin and box taper may be used, the lead may remain the same, and the thread cross sectional dimension may remain the same.

While a preferred embodiment of the invention has been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit of the invention. That being claimed is: 1. A threaded coaxial tubular connection for two tubular members suitable for connecting mine raise drill pipe, e.g. of the order of four to twelve inches in diameter, comprising a box on one end of one of said tubular members, said box having an internally tapered bore flaring toward the mouth of the box and merging at its smaller diameter end with the cylindrical inner periphery of said one tubular member, said box having a transverse end face adjacent the larger diameter end of said bore, the inner periphery of said box being unthreaded over a portion of its area extending from said end face toward the juncture of the tapered bore of the box with the cylindrical inner periphery of said one member, said unthreaded portion forming the mouth of the box, said box having double, modified buttress, helical threads projecting inwardly from said bore toward the axis of the members and extending from the mouth of the box toward said juncture of the tapered bore of the box and zthe cylindrical inner periphery of said one member, said threads projecting inwardly no farther than the cylindrical surface delined by said inner periphery of said one member so that the threads gradually reduce in height as they approach said junction until they vanish altogether at said junction, said threads having a lead between inch per foot and 1% inches and having a height in the range of ,4; inch to 7 and an average width of the same order of magnitude as the height, the pressure thread flank angle being of the order of to 20 degrees, the other thread flank angle being of the order of to 60 degrees,

a pin on an internal upset end of the other of said tubular members, tapering down toward the end of the pin, the large diameter end of said pin having a smaller diameter than the diameter of the outer periphery of said other tubular member forming a transverse shoulder adjacent the large diameter end of said pin adapted for engagement with said end face of the box,

said pin having double, modified buttress, helical threads similar to the threads on the box projecting outwardly from said pin away from the axis of the members and extending from a plane spaced axially from said shoulder toward the end of the pin, said pin and box threads being narrower than the spaces between the threads leaving space between the nonpressure thread flanks when the connection is made up,

the portion of the pin extending from the shoulder to the commencement of the pin threads constituting the pin root, the outer peri hery of the pin root being a tapered surface relieved below the taper of the threaded portion of the pin, said pin root lying opposite the mouth of the box when the connection is made up, the length of said pin root and box mouth being of the same order of magnitude as the lead of said threads,

said pin and box having a taper in the range of 1 inches per foot to 1% inches per foot, the distance from the box end face to the apex of the box thread crest cone being greater than the distance from the pin shoulder to the apex of the pin thread root cone and the distance from the box end face to the apex of the box thread root cone being greater than the distance from the pin shoulder to the apex of the pin thread crest cone so that when the pin shoulder and box end face are in engagement there are spaces between the pin and box thread crests and the adjacent portions of the box and pin peripheries, the last said spaces remaining even after the box has been made up on the pin beyond the initial engagement of pin shoulder and box end face by an axial travel of the order of 5% of the lead.

2. Combination according to claim 1 in which the thread surfaces are phosphatized and the connection has a breakout torque of the order of of the make-up torque.

3. Combination according to claim 1 in which the pressure flank angles of the threads are substantially 15 degrees, the non pressure flank angles of the threads are substantially 48 degrees, the lead is 1 inch, and the pin and box taper is 1 /2 inches per foot.

References Cited UNITED STATES PATENTS 1,879,856 9/1932 Peterson 285-333 x 2,111,196 3/1938 Texter 285-334 X 2,177,100 10/1939 Frame 285-334 3,047,316 7/1962 Wehring et a1. 285-334 3,050,3-18 8/1962 Van Der Wissel 285-334 3,129,963 4/1964 Robbins 285334 FOREIGN PATENTS 513,952 2/1955 Italy.

EDWARD C. ALLEN, Primary Examiner.

CARL W. TOMLIN, Examiner.

60 D. W. AROLA, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,355,192 November 28, 1967 Joseph A. Kloesel, Jr. et a1.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, lines 39 and 40, and column 7, line 46, cancel "per foot", each occurrence.

Signed and sealed this 13th day of January 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Edward M. Fletcher, J r.

Commissioner of Patents Attesting Officer 

1. A THREADED COAXIAL TUBULAR CONNECTION FOR TWO TUBULAR MEMBERS SUITABLE FOR CONNECTING MINE RAISE DRILL PIPE, E.G. OF THE ORDER OF TWELVE INCHES IN DIAMETER, COMPRISISNG A BOX ON ONE END OF ONE OF SAID TUBULAR MEMBERS, SAID BOX HAVING AN INTERNALLY TAPERED BORE FLARING TOWARD THE MOUTH OF THE BOX AND MERGING AT ITS SMALLER DIAMETER END WITH THE CYLINDRICAL INNER PERIPHERY OF SAID ONE TUBULAR MEMBER, SAID BOX HAVING A TRANSVERSE END FACE ADJACENT THE LARGER DIAMETER END OF SAID BORE, THE INNER PERIPHERY OF SAID BOX BEING UNTHREADED OVER A PORTION OF ITS AREA EXTENDING FROM SAID END FACE TOWARD THE JUNCTURE OF THE TAPERED BORE OF THE BOX WITH THE CYLINDRICAL INNER PERIPHERY OF SAID ONE MEMBER, SAID UNTHREADED PORTION FORMING THE MOUTH OF THE BOX, SAID BOX HAVING DOUBLE, MODIFIED BUTTRESS, HELICAL THREADS PROJECTING INWARDLY FROM SAID BORE TOWARD THE AXIS OF THE MEMBERS AND EXTENDING FROM THE MOUTH OF THE BOX TOWARD SAID JUNCTURE OF THE TAPERED BORE OF THE BOX AND THE CYLINDRICAL INNER PERIPHERY OF SAID ONE MEMBER, SAID THREADS PROJECTING INWARDLY NO FARTHER THAN THE CYLINDRICAL SURFACE DEFINED BY SAID INNER PERIPHERY OF SAID ONE MEMBER SO THAT THE THREADS GRADUALLY REDUCE IN HEIGHT AS THEY APPROACH SAID JUNCTION UNTIL THEY VANISH ALTOGETHER AT SAID JUCTION, SAID THREADS HAVING A LEAD BETWEEN 3/4 INCH PER FOOT AND 1 1/4 INCHES AND HAVING A HEIGHT IN THE RANGE OF 1/8 INCH TO 3/16" AND AN AVERAGE WIDTH OF THE SAME ORDER OF MAGNITUDE AS THE HEIGHT, THE PRESSURE THREAD FLANK ANGLE BEING OF THE ORDER OF 10 TO 20 DEGREES, THE OTHER THREAD FLANK ANGLE BEING OF THE ORDER OF 30 TO 60 DEGREES, A PIN ON AN INTERNAL UPSET END OF THE OTHER OF SAID TUBULAR MEMBERS, TAPERING DOWN TOWARD THE END OF THE PIN, THE LARGE DIAMETER END OF SAID PIN HAVING A SMALLER DIAMETER THAN THE DIAMETER OF THE OUTER PERIPHERY OF SAID OTHER TUBULAR MEMBER FORMING A TRANSVERSE SHOULDER ADJACENT THE LARGE DIAMETER END OF SAID PIN ADAPTED FOR ENGAGEMENT WITH SAID END FACE OF THE BOX, SAID PIN HAVING DOUBLE, MODIFIED BUTTRESS, HELICAL THREADS SIMILAR TO THE THREADS ON THE BOX PROJECTING OUTWARDLY FROM SAID PIN AWAY FROM THE AXIS OF THE MEMBERS AND EXTENDING FROM A PLANE SPACED AXIALLY FROM SAID SHOULDER TOWARD THE END OF THE PIN, SAID PIN AND BOX THREADS BEING NARROWER THAN THE SPACES BETWEEN THE THREADS LEAVING SPACE BETWEEN THE NONPRESSURE THREAD FLANKS WHEN THE CONNECTION IS MADE UP 